Indoor Bonsai — Soil, Watering and Grow Lights

Soil: The highest-quality bonsai soil includes lava rock, pumice and akadama in an average ratio of about 1/4 part each of lava rock and pumice to 1/2 part akadama. The best soil is tree-specific, so the amount of each component can vary slightly. These components also affect the watering schedule — adding more akadama increases water retention, meaning less frequent watering. You can make this work in your favor only if you properly research the quantity of each component in your mix.

Water retention test / watering: It is NOT a good idea to water when the soil still feels and looks wet. However, soil can sometimes look wet while actually being rather dry, so sticking your finger about 1 centimeter into the soil is the best way to judge. Soil should feel only slightly dry, not wet, to the touch.

Grow lights: Fluorescent lights should be kept ON 16–18 hours a day as a major light source, and can also supplement outdoor bonsai. Place them close to the canopy, about 8–10 inches away — burnt leaf-tip edges mean the lights are too near, while too far away fails to produce the best PAR. Choose the Kelvin rating by growth stage: 2700K CFL initiates flowering/budding, 6500K provides the spectrum for healthy foliage, and 5000K is best for the main period of growth. Fluorescent lights emit little heat, making temperature management easy and avoiding drying out the soil.

HID lights: Require proper ventilation, or the hot stagnant air can harm the bonsai. Keep HID lights about 20–24 inches from the trees, monitor regularly and reposition as needed.

LED lights: As a rule of thumb, keep 50W LED lights about 10 inches from the trees; use this as a reference to calculate the best distance for your system's power.